1. Field of the Invention
The present invention relates to a lightwave trransmission system and, in particular, to a lightwave transmission system of a balanced type which is adaptable for an optical digital interface for connecting between electronic devices.
2. Description of the Prior Art
In the prior art, the lightwave transmission system is used as an optical interface to transmit a digital signal from an electronic device to another electronic device.
As a conventional lightwave transmission system, an unbalanced type is used which is shown in, for example, a paper by T.L. MAIONE et al entitled "PRACTICAL 45-Mb/s REGENERATOR FOR LIGHTWAVE TRANSMISSION", THE BELL SYSTEM TECHNICAL JOURNAL, JULY-AUGUST 1978, p.p. 1837-1855 (Reference 1). In the unbalanced type lightwave transmission system, an electric digital signal is converted to an optical signal at a transmitter. The optical signal is transmitted through an optical fiber to a receiver. The receiver converts the received optical signal into an electrical signal as an output signal and maintains a constant output level by automatic gain control (AGC). The output signal is compared with a reference level to reproduce the electric digital signal. The reference I illustrates a one-way repeater comprising the receiver and the transmitter.
The AGC circuit comprises a peak detecting circuit for detecting a peak level of the output signal and an AGC signal generator responsive to the detected peak level for producing the AGC signal.
Generally speaking, digital signals are classified into a type such as an RZ signal having no DC component and another type such as an NRZ signal having a DC component.
When the digital signal has a DC component, it is hard to detect the peak level so that the AGC cannot be performed. Therefore, the conventional unbalanced lightwave transmission system cannot be used in an optical digital interface for transmitting the digital signal having a DC component.